1. Field of the Invention
The present invention relates to a piezoelectric sensor and a piezoelectric element.
2. Description of the Related Art
Examples of a piezoelectric sensor for detecting a deflection include a piezoelectric sensor described in Japanese Unexamined Patent Application Publication No. 2007-163230. In this piezoelectric sensor, a piezoelectric substance such as aluminum nitride is formed into a thin-film shape on one main surface of a film-shaped flat substrate made of a flexible polymeric material such as polyimide, and electrode layers are provided on both main surfaces of the flat substrate including the piezoelectric substance on one of the main surfaces.
This piezoelectric sensor, which is in the form of a very thin film and is sufficiently flexible, is used in close contact with a curved portion, for example, the inner surface of a tire. Thus, the piezoelectric sensor can detect the deformation of a tire caused by, for example, the change in the inflation pressure of the tire.
However, the piezoelectric sensor described in Japanese Unexamined Patent Application Publication No. 2007-163230 has the following problems.
The piezoelectric sensor described in Japanese Unexamined Patent Application Publication No. 2007-163230 includes a thin-film piezoelectric substance and is directly bonded to a tire by using an adhesive. As a result, the piezoelectric sensor detects, in addition to a signal generated by the deformation of a tire due to the change in the inflation pressure of the tire, various signals caused by, for example, extraneous vibration and impact directly transmitted from the tire at the same time. Accordingly, in order to accurately detect only the deformation of the tire due to the change in the inflation pressure of the tire, a complex signal processing circuit is needed.
Since the piezoelectric sensor described in Japanese Unexamined Patent Application Publication No. 2007-163230 is directly bonded to a tire by using an adhesive, a peripheral circuit such as a signal processing circuit provided around the piezoelectric sensor is connected to the piezoelectric sensor by using, for example, many lead wires. The number of wires in the tire is therefore large. Since the wires are long, the pattern of the wires is complex. It is, therefore, difficult to mount another electronic component (e.g., an IC) on the piezoelectric sensor.
While a film-shaped piezoelectric sensor can be made very thin, the piezoelectric sensor needs to have a large area to increase sensitivity, and becomes large in size. If a tire is punctured by a sharp object such as a nail, a piezoelectric sensor itself and a peripheral circuit are likely to be damaged. In this case, the deformation of a tire cannot be detected.